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kskunk

Member Since 1 May 2007
OFFLINE Last Active May 25 2012 9:34 AM

Topics I've Started

"Bus stuffing" like The Graduate.

Tue Jun 7, 2011 12:32 AM

Hi guys,

Thanks to Curt's great archiving work, we have the designer's notes on The Graduate peripheral for the 2600:
http://www.atarimuse...2600/a3000.html

They invented a 3-cycle "Bus Stuff" mode, to achieve an even faster TIA register update rate than the Harmony's 5-cycle/DPC+ "Fast Fetch" mode.

This works by loading Y with $FF at the beginning of the kernel, and then having the 6507 execute 3-cycle STY $REG instructions. At the critical moment when the $FF is being written, The Graduate hardware steps in and overdrives the the desired value on the bus. This avoids the extra 2-cycle LDA $VALUE used by Harmony.

Even though it seems pretty evil to overdrive the 6507's bus, the designers knew it was fairly safe because the NMOS 6507 used pull-up resistors to drive 1s on the bus, which could be grounded to 0s without overheating the 6507.

I'm sure this technique has been discussed before, but I couldn't find any actual hardware that could do it.

Has anyone tried "bus stuffing"? Can the Harmony do it, or are there resistors in the way?

- KS

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